The dehydration of alcohols using a heterogeneous alumina catalyst is a well known chemical transformation. Early work on this reaction showed that alcohols could be converted into their corresponding olefins, without extensive isomerization, if base treated alumina was used as a catalyst. See for example, Herman Pines et. al. in the Journal of the American Chemical Society, 1960, v82, at pages 2401 and 2471 and Herman Pines et. al. in the Journal of the American Chemical Society, 1961, v83 at pages 2847 and 3270. The authors treated alumina with sodium hydroxide, ammonia, methylammonia and pipyridine and found that, the stronger the base used, the smaller the degree of isomerization observed. Hence by rendering an alumina catalyst less acidic, isomerization of product olefins was suppressed.
The patent literature discloses several methods to produce olefins from both the dehydration of alcohols or the cracking of ethers using a base modified alumina catalyst.
For example, U.S. Pat. No. 4,234,752 to Phillips Petroleum Company discloses that linear or branched alcohols containing from 2 to about 20 carbon atoms can be converted to olefins by dehydration over a base modified gamma-alumina catalyst. In one embodiment, 3-methyl-1-butanol was converted to 3-methyl-1-butene with over 90% yield. Excessive acidic sites in the alumina were neutralized with sodium hydroxide or potassium hydroxide.
US Patent Application, 2003/0065233 to Kuraray Co. describes a process for making α-olefins from primary alcohols or ethers through elimination reactions catalyzed by a modified alumina catalyst. The alumina catalyst was modified with an organic amine.
Similarly, US Patent application 2006/0036121 to Oxeno discloses a base catalyzed process for the production of α-olefins from ethers (“1-alkoxyalkanes”). The catalysts used were alumina or zirconia catalysts which had been treated with alkali and/or alkali earth oxides. In a preferred embodiment 1-methoxyoctane is converted to 1-octene with 94.2% selectivity at a conversion of about 83.7%.
Cracking processes, such as those which convert ethers to α-olefins may play a significant role in the final stages of converting butadiene to 1-octene, an α-olefin of significant commercial utility, particularly to the plastics, surfactants and lubricants industries.
For example, WO 92/10450 describes a process in which 1,3-butadiene is converted first to 2,7-octadienyl ether over a palladium catalyst in the presence of methanol. The 2,7-octadienyl ether is then hydrogenated to 1-methoxyoctane which is cleaved over alumina to provide 1-octene. Alternatively, butadiene may be reacted with a carboxylic acid instead of methanol in the first step according to EP 0,440,955. The selectivity of the ether cleavage step in each of these processes is poor at high conversions, due to acidic alumina sites which not only catalyze the ether cleavage reaction, but also contribute to isomerization of the product α-olefins to internal olefins.
There remains a need for new catalysts which crack ethers into 1-olefins with an improved combination of selectively and conversion.
The current invention provides a process in which ethers are converted to α-olefins with high selectivity at high conversions using an alumina catalyst which has been treated with a rare earth metal oxide.
The current invention also provides modified alumina catalysts in which selectivity to 1-olefins as well as conversion rates are relatively insensitive to the loading of the rare earth metal oxide. Hence, preparation of the inventive catalysts is facile and requires little optimization.